Within electrochemical measurements a potentiometric measurement is a commonly used measurement technique. Here, a potentiometric measurement is illustrated as a typical example of an electrochemical measurement. In a potentiometric measurement a potentiometric sensor, i.e. an indicator electrode, is used to determine certain chemical properties, e.g. the concentration of some components of the analyte gas or the analyte solution. A potentiometric sensor is a type of electrochemical sensor that measures the electrical potential in zero current conditions. The signal is measured as the potential difference (voltage) between the indicator electrode and the reference electrode. The potential of the indicator electrode depends on the measured chemical properties e.g. the concentration of the analyte in the gas or solution phase.
In a potentiometric measurement the chemical information, i.e. the activity of ions is translated into easily measurable electrical potential. In a typical potentiometric measurement the measured cell potential (Emeas) consists of the indicator electrode potential (Eind), reference electrode potential (Eref) and the liquid junction potential (Ej):Emeas=Eind+Eref+Ej 
In potentiometric measurement the role of the reference electrode is to maintain a constant potential independent of the sample composition. In a typical potentiometric measurement the indicator electrode gives a selective response corresponding to the concentration of the analyte to be measured according to:E=const+S ln ai E=const+S ln [ai+ΣKijpot(aj)zi/zj+L], where E is the measured potential, ai is the activity of the measured analyte, S is the slope of the linear part of the calibration curve, Kij is the selectivity coefficient and L is the detection limit. In a potentiometric measurement the reference electrode is an indispensable and crucial component both in potentiometry and open-circuit sensor technology as well as a reference point in amperometric measurements.
Potentiometric ion sensors including ion-selective electrodes (ISEs) are an important subgroup of electrochemical sensors. Ion-selective electrodes are characterized by small size, portability, low energy consumption, and low cost, which are attractive features concerning practical applications.
In the following, the prior art will be described with reference to the accompanying drawings of FIGS. 1 to 2, of which:
FIG. 1 shows one embodiment of an arrangement for an electrochemical measurement according to prior art; and
FIG. 2 shows another embodiment of an arrangement for an electrochemical measurement according to prior art.
FIG. 1 shows one embodiment of an arrangement for an electrochemical measurement according to the prior art. The arrangement for an electrochemical measurement according to the prior art comprises an ion-selective indicator electrode 1 and an external reference electrode 2. Both the ion-selective indicator electrode 1 and an external reference electrode 2 are placed in an analyte solution 3 the chemical properties of which solution 3 are to be measured. The potential difference between the electrodes 1 and 2 is measured. The potential produced is the sum of several individual potentials. Potential-determining processes always occur at the phase boundaries, e.g. between the solution 3 and the ion-selective membrane of the indicator electrode 1.
The ion-selective indicator electrode 1 of an arrangement for a electrochemical measurement according to the prior art comprises an internal reference element 4, an internal electrolyte solution 5 and an ion-selective membrane (ISM) 6. The external reference electrode 2 according to the prior art comprises a reference element 7 e.g. a reference electrode wire 7, a reference electrode solution 8 and a liquid junction 9.
As the potential in the arrangement for an electrochemical measurement according to the prior art is measured the external reference electrode 2 maintains a constant potential. Likewise, the ion-selective indicator electrode 1 gives a constant potential between the internal reference element 4 and the internal electrolyte solution 5 and also gives a constant potential between the internal electrolyte solution 5 and the ion-selective membrane 6. The ion-selective indicator electrode 1 according to the prior art is constructed so that if the ion to be measured is present in the analyte solution 3 then this ion can permeate the ion-selective membrane 6 of the ion-selective indicator electrode 1. This alters the electrochemical properties of the membrane and causes a change in potential.
Currently, ion-selective electrodes based on polymeric membranes containing neutral or charged carriers (ionophores) are available for the determination of a large number of inorganic and organic ions. Furthermore, during the past decade, the chemical sensing abilities of ISEs have been improved considerably. This can be attributed to several important findings, such as the considerable improvement in the lower detection limit of ISEs, new membrane materials, new sensing concepts, and deeper theoretical understanding and modeling of the electrochemical response of ion-selective electrodes.
FIG. 2 shows another embodiment of an arrangement for an electrochemical measurement according to the prior art. Another embodiment of an arrangement for an electrochemical measurement according to the prior art comprises two ion-selective indicator electrodes 10, 11 and an external reference electrode 12. One potential difference related to the first ion-selective indicator electrode is measured between the electrodes 10 and 12 and another potential difference related to the second ion-selective indicator electrode is measured between the electrodes 11 and 12.
The reference electrode 12 is an indispensable and crucial component in potentiometry and open-circuit sensor technology as well as a reference point in amperometric measurements. The failure of the reference electrode 12 means the failure of the entire system, so that none of the indicator electrode measurements can be collected. Thus, the quality of the reference electrode 12 is critical in electrochemical measurements, especially these where multi-parameter analyses are performed.
There have been alternative prior art electrochemical reference electrodes designed and studied in the past two decades. In one prior art electrochemical reference electrode design study there are equitransferent salts dispersed in a polymer or other solid. In one prior art embodiment polyvinyl resin doped with a very large amount of KCl has been used. Despite the heavy salt loading and large surface area in contact with the liquid sample, the reported leakage of KCl into the sample solution is less than what occurs with conventional ceramic frit junctions. The junction potential is quick to stabilize and relatively constant with time even in media with a very low ionic strength.
In another prior art electrochemical reference electrode design study there are other polymers or resins used, e.g. pressed Al2O3-PTFE, urea-formaldehyde, poly(methyl methacrylate)-propylene carbonate and/or polyester resin. Also in a third prior art electrochemical reference electrode design study an all solid reference electrode was introduced, consisting of a sintered Ag/AgCl mixture embedded in solid remelted KCl. Although these prior art concepts are rather different on the surface, the unifying factor is the controlled release of equitransferent salt from either a polymer material or a dense glass or ceramic sinter.
Unfortunately, all of the above-mentioned prior art electrochemical reference electrode design studies displayed relatively high electrical resistances (about 1-500 MΩ) for the reference electrode and it was reportedly not possible to get reproducible results.
Conventional prior art electrochemical reference electrodes have many problems and disadvantages. The prior art electrochemical reference electrodes are complicated and expensive to manufacture. The prior art electrochemical reference electrodes need to have the inner solution refilled and the liquid junction kept clog free and are therefore troublesome and maintenance intensive. The prior art electrochemical reference electrodes typically only work in upright position. Also the prior art electrochemical reference electrodes may easily foul or leak the internal solution into the sample. Also the prior art electrochemical reference electrodes are problematic as they have a separate physical body, are mechanically not very robust due to mainly glass construction, and are very difficult to miniaturize.
As mentioned above, there are a lot of deficiencies in the current reference electrodes. There is a clear demand in the market for a new type of electrochemical reference electrode that would be better and more efficient than the current prior art electrochemical reference electrode solutions. Likewise, there is a clear demand in the market for a new type of an arrangement for an electrochemical measurement that would be better and more efficient than the current prior art electrochemical measurement arrangement solutions.